Turbo compressor surge is mild to violent air instability that occurs in a turbo compressor when asking for high boost at low Mass Air Flow (MAF). Under these conditions, the compressor wheel loses its grip or ability to compress this low air mass to the pressure requested by the engine calibration. Suddenly, the pressure in the boost tube leaving the compressor wheel is higher than the air exiting the compressor wheel. The flow reverses through the wheel and into the intake air system! This results in moderate to huge swings in boost pressure and causes flow rate fluctuations where the direction of airflow through the compressor changes from forward to reverse.

Mild compressor surge can be almost silent but its effects can be measured with proper instrumentation. Even in mild form, surge causes hotter than normal boost air as the flow reversal causes the boost air to pass through the compressor repeatedly before reaching the engine. Surge also overheats the compressor wheel, shock loads the turbo thrust bearing, and requires more exhaust pressure to drive the turbine. This reduces throttle response, kills your turbo life, performance, and fuel economy.

Compressor “deep surge” is audible and more violent. When this happens, the amplitude of boost pressure and MAF fluctuations increase dramatically as does the boost air temperature. Hot reverse flow boost air blows back into the intake pipe every time you hear a pulse or whoosh. Then the air goes back through the compressor, heating it even more! Running your turbo in deep surge can destroy the compressor wheel and beat the thrust bearing to death. Deep surge dramatically diminishes performance, fuel economy, and can kill your turbo and engine. And, you can’t remove surge by “tuning it out” without further diminishing your engine’s performance.

To prevent surge, various surge control system are employed to reduce the impact of the damaging reverse flow pulse and reduce the noise. On the Duramax L5P a slot in the compressor inlet bore feeds the pulses into an annular volume shaped somewhat like a donut. Altering or removing this geometry defeats the anti-surge system entirely, amplifying the damage.

Here the pulse is diffused and the temperature and pressure spike is reduced. The boost air then makes a U-turn and re-enters the intake air through a pathway that is defined by the marriage of the turbo compressor housing with a correctly machined inlet elbow.

Of the competitive turbo inlet elbows tested, all cause surge. Some claim to have anti-surge rings that are nothing more than decorative. They do not help the air make a smooth U-turn. Others who claim to have anti-surge rings more closely resembling the stock elbow, do not have the precise geometry required to prevent surge. Tests prove that being off a few thousandths of an inch results in surge. There are currently two on the market with identical geometry as the stock anti-surge ring. However, they flow less than stock because the elbows are made of welded pieces of tubing. Air doesn’t like corners.

Other brands offer a fabricated inlet elbows that causes surge. To fix this surge, 3rd party aftermarket tuners offer an anti-surge ring that bolts on between the turbo and inlet. The irony is that after the bolt-on anti-surge ring is attached, the elbow flows less than stock.

– Gale Banks | President & Chief Engineer